World of Rollercoaster Tycoon - only £9.99
Contains the titles:
Rollercoaster Tycoon 2
Rollercoaster Tycoon 2 Expansion Pack: Wacky Worlds
Rollercoaster Tycoon 2 Expansion Pack: Time Twister
Rollercoaster Tycoon 3
Rollercoaster Tycoon 3: Soaked
Rollercoaster Tycoon 3: Wild.
This is a big drop in price, the previous best price was £17.99!
You can also get 5% off using either Quidco or the voucher code 5OFFWEB (this expires on monday though, according to the voucher posting here). It wont let you use both.
Also, another good deal I spotted in their "massive clearout" sale is Lego Star Wars II: The Original Trilogy (PC) for £4.99 (best price i've found). I have ordered the two games myself and have had 75p Quidco tracked.